Add mode to disallow unprofitable leases

**Is your feature request related to a problem? Please describe.**

Currently you can need to guestimate if your lease will profitable or not dependent on on-chain fees and the execution fee vs the rate you get.

**Describe the solution you'd like**

It would be nice if there way a flag in the config to only sign leases that net a profit (maybe even a min profit). This should remove some of the guessing game needed and give users more peace of mind that they won't lose money besides on the back half when closing the channel.

Start by locating the configuration handling and lease-signing flow. Define how execution fees, on-chain fees, lease rates, and any minimum-profit threshold determine whether a lease is signed. Done means the requested mode is configurable and its profitability behavior is covered by tests.

Written by the indexing model from the issue text.